TopLink源代码分析 JTA的部分技术实现细节

TopLink源代码分析 JTA的部分技术实现细节

[什么是JTA]

Java事务API（JTA；Java Transaction API）和它的同胞Java事务服务(JTS；Java TransactionService)，为J2EE平台提供了分布式事务服务。一个分布式事务（distributedtransaction）包括一个事务管理器（transaction manager）和一个或多个资源管理器(resource manager)。一个资源管理器（resource manager）是任意类型的持久化数据存储。事务管理器（transaction manager）承担着所有事务参与单元者的相互通讯的责任。下图显示了事务管理器和资源管理的间的关系。

　　JTA事务比JDBC事务更强大。一个JTA事务可以有多个参与者，而一个JDBC事务则被限定在一个单一的数据库连接。下列任一个Java平台的组件都可以参与到一个JTA事务中：

　　.JDBC连接

　　.JDO PersistenceManager 对象

　　.JMS 队列

　　.JMS 主题

　　.企业JavaBeans（EJB）

　　.一个用J2EE Connector Architecture 规范编译的资源分配器。

简单的说，JTA实现了在分布式系统中对单一事务的提交和管理。

 

[源代码分析]

第一步：当连接数据库的时候，前期的准备函数，这里面都发生了什么？

/**      * INTERNAL:      * This method includes all of the code that is issued before the datasource      * is connected to.      */     protected void preConnectDatasource(){         //Bug#3440544 Check if logged in already to stop the attempt to login more than once         if (isLoggedIn) {             throw ValidationException.alreadyLoggedIn(this.getName());         }         this.platform = null;         if (isInProfile()) {             getProfiler().initialize();         }         updateProfile(SessionProfiler.LoginTime, new Date(System.currentTimeMillis()));           // Login and initialize         getEventManager().preLogin(this);         //setup the external transaction controller         getServerPlatform().initializeExternalTransactionController();         log(SessionLog.INFO, null, "topLink_version", DatasourceLogin.getVersion());         if (getServerPlatform().getServerNameAndVersion() != null) {             log(SessionLog.FINE, null, "application_server_name_and_version", getServerPlatform().getServerNameAndVersion());         }     }

 

第二步：上面的程序中，有这样一行代码

//setup the external transaction controller      getServerPlatform().initializeExternalTransactionController();

它是干什么的呢？？？

public void initializeExternalTransactionController() {         this.ensureNotLoggedIn();           //BUG 3975114: Even if JTA is disabled, override if we're in CMP         //JTA must never be disable during CMP (WLS/Oc4j)         if (!isJTAEnabled() && !isCMP()) {             return;         }         //BUG 3975114: display a warning if JTA is disabled and we're in CMP         if (!isJTAEnabled() && isCMP()) {             AbstractSessionLog.getLog().warning("jta_cannot_be_disabled_in_cmp");         }         //check if the transaction controller class is overridden by a preLogin or equivalent,         //or if the transaction controller was already defined, in which case they should have written         //a subclass. Show a warning         try {             if (getDatabaseSession().getExternalTransactionController() != null) {                 this.externalTransactionControllerNotNullWarning();                 return;             }             ExternalTransactionController controller = null;             if (PrivilegedAccessHelper.shouldUsePrivilegedAccess()){                 try {                     controller = (ExternalTransactionController)AccessController.doPrivileged(new PrivilegedNewInstanceFromClass(this.getExternalTransactionControllerClass()));                 } catch (PrivilegedActionException exception) {                     Exception throwableException = exception.getException();                     if (throwableException instanceof InstantiationException) {                         throw ValidationException.cannotCreateExternalTransactionController(getExternalTransactionControllerClass().getName());                     } else {                         throw ValidationException.cannotCreateExternalTransactionController(getExternalTransactionControllerClass().getName());                     }                 }             } else {                 controller = (ExternalTransactionController)PrivilegedAccessHelper.newInstanceFromClass(this.getExternalTransactionControllerClass());             }             getDatabaseSession().setExternalTransactionController(controller);         } catch (InstantiationException instantiationException) {             throw ValidationException.cannotCreateExternalTransactionController(getExternalTransactionControllerClass().getName());         } catch (IllegalAccessException illegalAccessException) {             throw ValidationException.cannotCreateExternalTransactionController(getExternalTransactionControllerClass().getName());         }     }

乖乖~~~~~这么长的函数，鬼知道它做的是什么。。。。

不过，貌似它里面有这样一行话。

getDatabaseSession().setExternalTransactionController(controller);

乖乖~~~~原来是用来启动JTA事务管理控制器的。。。快点去看看它干了什么。。

 

第三步：啊哈，奥秘在这里

public class SunAS9TransactionController extends JTATransactionController {     // Use "java:appserver/TransactionManager" instead of java:pm/TransactionManager     // as the former one is available in ACC as well as server.     // See com.sun.enterprise.naming.java.javaURLContext in GlassFish appserv-core     public static final String JNDI_TRANSACTION_MANAGER_NAME = "java:appserver/TransactionManager";       public SunAS9TransactionController() {         super();     }       /**      * INTERNAL:      * Obtain and return the JTA TransactionManager on this platform      */     protected TransactionManager acquireTransactionManager() throws Exception {         return (TransactionManager)jndiLookup(JNDI_TRANSACTION_MANAGER_NAME);     } }

同志们看见了吗~~~~

public static final String JNDI_TRANSACTION_MANAGER_NAME = "java:appserver/TransactionManager";

 

搞了半天，原来它用JMX启动了一个交易服务来管理JTA~~~

？？？什么是JMX~~~~~

请看下回分解。。。